Cosmology and high redshift galaxies

Probing Cosmic Microwave Background anomalies
N. Bartolo

The Cosmic Microwave Background (CMB) radiation is a privileged laboratory to study the initial conditions of the universe, its composition and evolution. It has allowed to define with extreme precision the standard cosmological model, which has been confirmed by a variety of other cosmological observations on a large range of scales.
There are however several statistical anomalies in the properties of the CMB fluctuations w.r.t to such standard model, especially on the largest angular scales. Even though their statistical significance is not high, such anomalies are interesting in that they might reveal new physics beyond the standard
cosmological model (e.g. due to a relic breaking of isotropy on the largest angular scales). Systematics might be a possibility, still up to now all the explanations in this sense have not been satisfactory, and moreover such anomalies have been reported independently both by the WMAP and the Planck satellite, two instruments with different systematics.
The aim of this PhD project is twofold. On the one hand an investigation of possible cosmological effects able to explain such anomalies will be developed. Possible scenarios to be analyzed include both early universe conditions and low redshift evolution.  On the other hand, we aim at investigating alternative observables able to better characterize the nature of these anomalies (e.g., fully exploiting the CMB polarization information, also in view of future CMB experiments) or observables related to future galaxy, radio, and gravitational-wave datasets, such as Euclid, SKA and Lisa.

Discovering New Populations of Astronomical Objects
E. Bottacini

The high-energy sky probes the most violent events in the Universe. One of the most interesting discoveries are variable and transient astronomical objects linked to gravitational wave events and to possible neutrino detections. For the latter we are still missing a clear evidence. The PhD candidate will analyze observations of NASA’s Fermi satellite focusing on the time domain, thereby discovering new source populations. The aim is to study the populations in the a multimessanger and multwavelength framework.
More precisely, the Fermi satellite scans the entire sky in 3 hours, which makes it an ideal sky monitor for variable and transient sources. Time dependent and spectral analyses allow for discovering new sources that go undetected in normal survey studies. The new source populations will be explored on statistical basis and in case studies applying multiwavelength and multimessanger approaches.
The ideal PhD candidate will learn how to analyze observations of the Fermi mission. For analyses regarding the time domain further programming in Python and C are required.
For more information: eugenio.bottacini[at]unipd.it or visit www.TheBotta.com

The Cosmological Evolution of AGN from High to Low Redshift
E. Bottacini, A. Franceschini

Sky surveys are at the core of astronomy. While currently flying high-energy missions continue their surveys, specific issues related to the understanding of the cosmological evolution of AGN call for more sensitive observations. In this project the PhD candidate is expected to perform a high-energy survey for which we have developed a new spectro-imaging survey technique that allows reaching a better sensitivity and that involves major high-energy missions by NASA and ESA including Chandra, XMM-Newton, Swift, INTEGRAL, and Fermi. Major international collaborations are involved. The resulting sample of AGN will be used to study the cosmological evolution of supermassive black holes in AGN.
In more detail, the non-thermal emission detected by high-energy missions is the footprint of activity in galaxies. Results from current surveys from high-redshift (z~3) to the local Universe show tensions in the evolution study of AGN. Further complications are brought to light by population synthesis models unable to correctly reproduce these surveys. To reconcile data and models a more sensitive survey is needed that detects the sources from high to low redshift that is able to pick up a rapid evolution of AGN.
The ideal PhD candidate is expected to learn to program in Python and C. Furthermore, the candidate is willing to write observing proposals for space missions.
For more information: eugenio.bottacini[at]unipd.it or visit www.TheBotta.com

Finding the sources that reionized the Universe
Andrea Grazian, Alberto Franceschini

Hydrogen Reionization is the last and major transition phase of the baryonic matter in the Universe. The first sources of HI ionizing photons were able to end-up the so-called ``Dark Ages'', cleaning the fog by neutral hydrogen and producing the widespread metagalactic ionizing background. It is now well established that the epoch of Reionization happened relatively late (z~7), rapidly (dz<1), and through a patchy and strongly inhomogeneous process.
Identifying the long time sought-for population that significantly contributed to the epoch of Reionization (EoR) is still an open and very debated problem of modern Cosmology. It is a widespread opinion that hydrogen Reionization in the Universe is mainly driven by star-formation activity in primeval galaxies, with a minor role of active galactic nuclei at high redshift. Recent observations, however, challenge this notion, indicating a number of issues related to a galaxy-driven Reionization scenario.
Recently, a revival of the role of bright QSOs and faint AGNs in the epoch of Reionization is progressively emerging, thanks to recent discoveries by our team. In particular, our group is currently studying the contribution of QSOs and AGNs at high-z to the ionizing background, by investigating the luminosity function and space density of these sources, estimating the Lyman Continuum escape fraction of bright QSOs and faint AGNs, and measuring the mean free path of ionizing photons close to the Reionization epoch. These analyses are
suggesting a dominant contribution of AGNs to the ionizing background up to z~5 and a possible important contributions up to z~6.
This PhD project will give the opportunity to analyze a large amount of data, thanks to deep multi-wavelength imaging with HST, Spitzer, and Chandra in wide areas of the sky (e.g. CANDELS, COSMOS), corroborated by deep spectroscopic observations with VLT, LBT, and Magellan telescopes. Observing proposals at VLT and LBT are in preparation and observing runs at these major facilities are possible. Comparisons of the observations with state-of-the-art
simulations are in progress, with deep connections with developers of numerical simulations of the Reionization phase of the Universe. This project will give also the opportunity to analyze, in the near future, the first James Webb Space Telescope data already approved on two large extragalactic fields, under the Early Release Science (ERS) initiative of JWST. In the long term, future workhorse facilities (e.g. Euclid and ELTs) will revolutionize the study of high-z, faint AGNs and their contribution to the Reionization of the Universe.

The Diverse Progenitors of Supernovae of Type Ia
L.Greggio

The nature of the progenitors of SNIa is still an unsettled question. While there is general consensus that the explosions originate from
the ignition of nuclear fuel under degenerate conditions in a White Dwarf (WD) member of a close binary system, the evolutionary paths leading to the event are controversial. Two competing scenarios are considered, one in which the donor star is a White Dwarf (double degenerate), and one in which it is a living star (single degenerate). In addition, explosion can occur when the WD mass has reached the Chandrasekhar limit, or when a sufficiently massive Helium layer has accumulated on top of the WD and detonates (sub-Chandra explosions). In the recent years more and more evidence for diversity of the spectrophotometric properties of SNIa’s has accumulated. Peculiar events, like Superchandra explosions, subluminous events, or the intriguing class of Type Iax Supernovae have supplemented the class of normal SNIa events in a non negligible proportion. The diversity of spectrophotometric propeties of SN of type Ia is a challenge for their use as distance indicators in cosmological applications. The aim of the thesis is to provide
a comprehensive overview of this diversity by collecting data from the literature, with special attention to the more exceptional events. Existing models for the SNIa explosions will provide a framework to develop associations between evolutionary paths and SNIa subtype. These will be used to compute the expected diversiy of events in galaxies with different star formation histories, to be tested against observational data. The thesis will finally include the discussion of the future prospects for this kind of analysis using the vast database provided by the LSST survey.

Supermassive black holes accreting at high rates: implication for cosmology
P. Marziani
M. D’Onofrio
P. Mazzei

Quasars are among the most luminous sources in the Universe. There are still many enigmas on their inner workings, and a large
spread in luminosity has hampered their use for cosmological studies. However, several recent works suggest that it may be possible to exploit a particular class, quasars due to massive black holes accreting at high rates, as cosmological distance indicators. The student will start from the definition and analysis of a large sample of highly-accreting quasars in terms of optical UV and soft-X-ray properties from survey and archival data (SDSS, BOSS, MAST, etc.) and already-collected Gran Telescopio Canaries observations. Through a detailed study of the optical and UV spectral properties, the project will lead to results on the quasar physical and structural parameters: black hole mass, luminosity Eddington ratio, metal content, and accretion disk wind dynamics, among them. Results will then be applied to derive redshift-independent luminosities and hence of the principal cosmological parameters with a proper analysis of statistical and systematic effects. The project involves collaborations with researchers at major institutes in  Spain and Mexico. In 3 years the student will acquire a culture on important theoretical and phenomenological aspects of quasars, the ability to analyze and measure spectroscopic multifrequency data as well as to apply methods commonly used in observational cosmology. The prospect of the thesis is to give a relevant contribution to a frontier field that will open further lines of research on active galactic nuclei and in observational cosmology.

HI-MIGHTEE: The first statistical census of HI content in galaxies at cosmological distances
G. Rodighiero, P. Cassata

Neutral gas is the most important component of the Universe since the epoch of Reionization.
A direct spectroscopic detection of atomic hydrogen (HI) is available only in the very local Universe,  showing that HI can be extended well beyond the optical radius of extended nearby galaxies. At higher redshift, only molecular gas can be detected in galaxies (in the form of carbon monoxide, CO) thanks to interferometric telescopes at radio millimeter frequencies.
The baryonic cycle of gas, stars and dust is parametrized through the evolution of empirical scaling relation of galaxies, that regulate their growth in terms of assembling of stellar mass and quenching mechanisms. The role of HI is, however, completed neglected in the current view of galaxy formation and evolution models.
While recent results on the local Universe seems to indicate an important role of HI in the galaxy lifecycle, we propose (as a PhD thesis project  in our group) to study for the first time in a statistical way the real impact of HI on galaxy evolution up to z=0.5.
This will be performed in the context of our collaboration with the MIGHTEE survey
(https://www.sarao.ac.za/science-engineering/meerkat/observers/observing-programme/large-survey-projects/), an ongoing observational program with the MeerKat radio telescope located in South Africa. MeerKat is a precursor of the major SKA  project, and it wil thusl allow the student to pave the way to the future all sky survey that will performed by the SKA.
The student will be involved in the analysis and exploitation of the MIGHTEE data, performing also statistical tools to study through stacking analysis the faint HI signal in large sample of galaxies.
He is expected to visit the South African Institute in Cape Town to be fully involved in the Consortium activities.
The second part of the Thesis will include the development of a theoretical cosmological model to interpret our results in the context of galaxy formation and evolution.

Euclid: galaxy formation and evolution from spectroscopic surveys
G. Rodighiero, P. Cassata

We are involved at different levels in the Euclid consortium. Our work is mostly related to the scientific validation and exploitation of the spectrophotometric surveys in the context of galaxy formation and evolution scenario, in particular for what concerns the star-forming population.
We offer a Thesis to collaborate in the generation of spectral libraries that will be used by the Consortium team to understand the performances of the
Euclid mission.
A thesis related to Euclid science can be largely tuned and discussed according to the interests of the student.

TeV astronomy, the problem of cosmic opacity at TeV photon energies, and the cosmic IR background

A. Franceschini

Observations of very high energy (TeV) photons are now routinely possible with Cherenkov telescopes. The team proposing the thesis collaborates with colleagues of the Physics Department involved in the MAGIC consortium. The thesis will concern the important question of the cosmological opacity of the universe to TeV photons due to their interaction with those of the cosmic optical and IR backgrounds (photon-photon collisions). The topic will not only address all the physical problems related with this phenomenon, but also will exploit TeV observations of BLAZARs over a wide redshift range to constrain the intensity of the extragalactic radiations and compare them with direct determinations and their model interpretation.

H-ATLAS: co-evolution of Black Hole accretion rate and Star-Formation in galaxies across cosmic times
G.  Rodighiero, A. Franceschini

The self-regulation of host galaxies and their central supermassive black holes (BHs) is often invoked to explain the local scaling relations between BH mass and galaxy properties. In the last years it has also been shown that this correlates mainly with velocity dispersion, a relation that can be hardly explained without feedbak effects. As a consequence, determining the relative roles of the various processes that drive the coevolution of BHs and galaxies has emerged as a key goal of current astrophysics research.The emerging observational framework supports the idea that mergers and/or secular processes should play a major role in fuelling both star formation and SMBH growth in a large majority of galaxies displaying moderate nuclear activity.
The student will be involved in an extensive exploitation of the photometric and spectroscopic H-ATLAS data-set (the largest far-Infrared Herschel survey). In particular he/she will perform an SED fitting to a complete mass selected sample of star-forming galaxies in the low-redshift Universe. The adopted code (Magphys) performs an energetic balance between UV absorbed and IR re-emitted radiation, and it provides the total star formation rate (SFR) and the intrinsic AGN luminosity (i.e. proportional to the black hole accretion rate) of galaxies.  In the thesis the candidate will then integrate these two physical quantities to trace the evolution of black hole masses and the stellar mass of their hosts. This project forsees a collaboration with the theoretical group of the Southampton University (in particular prof F. Shankar), where the student is supposed to spend a period to implement a phenomenological and semi-analytical model to interpret the data (in particular to understand the actual roles of host stellar mass/velocity dispersion and BH mass)  and put them into a cosmological context.

SHARDS: The bending of the star-forming galaxies Main Sequence: What is the major physical process driving the evolution of massive galaxies?

G. Rodighiero, C. Mancini, A. Franceschini, A. Renzini

At z<1.5 the stellar mass/Star-Formation Rate (SFR) correlation of star-forming galaxies (i.e., the so-called Main Sequence, MS) is not linear at all masses, but it bends at the high-mass end, due to a population of sources with reduced SFR at such high stellar mass. The fact that such objects host large bulges has recently suggested that the internal formation of the bulges was the late event that induced massive galaxies to quench.
However, in a recent work (Mancini et al. 2017, in prep.) an accurate bulge/disc decomposition performed in 7 optical/near-IR HST bands for 16 galaxies below the main sequence at 0.5<z<1 showed that while the bulges hosted in these systems are virtually all maximally old, with ages approaching the age of the Universe at the time of the observation, disks are very young (0.3-1.5 Gyr). If confirmed, these results would suggest that the bending of the MS is likely due to rejuvenation events, caused by the interaction of passive massive galaxies with smaller gas-rich systems.
During the thesis, the candidate will perform a bulge/disk decomposition (with Galfit and similar tools) on the 24 contiguous medium-band maps from the SHARDS programme (Perez-Gonzalez et al 2013), covering the whole optical wavelength range, between 5000 and 9500 A. He/she will then apply a detailed SED fitting procedure to the SED of the sources, deriving with improved spectral resolution (R~50)  the age of the bulge and disc  components. The analysis will be also extended to a larger sample, including all the main sequence galaxies at 0.5<z<1 in the GOODS-N field, and to higher-redshift galaxies in the ongoing SHARDS programs in the HST Frontier Fields.

SHARDS: a comprehensive study of Cosmic Star Formation up to redshift z~2
G. Rodighiero, L. Rodriguez-Munoz, C. Mancini, A. Franceschini

In the last decades, the cosmic star formation (SF) history of the Universe has been thoroughly studied to  better understand galaxy formation and evolution. In this context, emission lines (ELs; i.e., H-alpha) reveal as some of the most direct SF tracers.  We propose to carry out a comprehensive study of emission line galaxies (ELGs) up to redshift z~2 exploiting the spectro-photometric Survey for High-z Absorption Red and Dead Sources (SHARDS, Pérez-González et al. 2013), an ESO/GTC Large Program. SHARDS covers GOODS-N field (~130 arcmin^2) and provides deep photometry in 25 medium-band filters (reaching magnitude ~27, at 3 sigma) throughout the wavelength range 5000-9500 A.  The power of using narrow/medium-band filters for the detection of ELGs (i.e., star-forming) has already been proved in previous studies. In this project, [OII]3727A emitters will be detected in the redshift range between ~0.3 and ~2.4. With this sample, the student will compute the evolution of the EL luminosity functions, and their contribution to the cosmic SF rate density.
The candidate will be further involved in the ongoing SHARDS observations in the HST Frontier Fields and for this part of the project, he/she is expected to spend a period at the UCM (Madrid; Prof. Pérez-González) to work on GTC observations and related data reduction and analysis. The UV to far-IR photometric datasets and spectroscopic data available on GOODS-N will allow the student to perform a high quality characterization of these star-forming galaxies using a SED-fitting methodology, and a panoramic perspective on the SF activity throughout cosmological timescales.

Pinpointing the origin of high-redshift star-forming clumps and their role in galaxy evolution
A. Zanella

How do galaxies build up their stellar mass? How do they evolve, change structure, and grow a bulge as time passes? Answers to these fundamental questions are currently still missing.
Observations of galaxies at the peak epoch of cosmic star formation (redshift z ∼ 1 − 3) have revealed that they usually have irregular morphologies dominated by kpc-size, massive, and blue star-forming regions (clumps), rarely observed in the local Universe. There is currently no consensus on the origin and fate of clumps. Also their role in galaxy evolution is still highly debated.
To explore these open questions we have been recently awarded observing time on the Very Large Telescope, one of the world's most advanced observatories, with the instrument HAWKI (near-infrared imaging with adaptive optics). By using these state-of-the-art observations, together with ancillary available data (e.g. from Hubble Space Telescope), the student will investigate what is the fraction of clumpy galaxies at z ~ 2, how clumps form, how long they survive, and whether they contribute to the growth of galaxy bulges. He/She will learn how to reduce and analyze near-infrared photometric observations, and how to interpret multi-wavelength datasets. This study will prepare the ground for follow-up observations with cutting-edge telescopes such as ALMA and the James Webb Space Telescope. The student will be inserted in an international collaboration and will be encouraged (and guided) to write observing time proposals for these observatories. This project will also set the stage for future observations with the Extremely Large Telescope, the largest telescope in the world, now under construction in the Atacama desert (Chile). The student will have the possibility to spend part of the PhD abroad (e.g. at the European Southern Observatory, working with Joël Vernet).
For questions and a more detailed discussion feel free to contact me:
Anita Zanella – anita.zanella@inaf.it